Head gasket??

[popslavaz]

Freshening up my 383 out of my ‘71 RR. I would like to use thin steel head gaskets to raise compression a tad.Motor has stock type KB flat tops with valve reliefs.No cutting ever on block or heads.The stock OEM cam had .450-.458 lift and came with steel gaskets.My cam that was in it with Victor 3464 .042 gaskets .474-.474 lift.I am useing the same cam but would like to use Mopar 4286754 .020 gaskets.See any problem going to the thinner gaskets??? POPS

 

[oldbee]

Well “pops” , i don’t think I’d worry about it; but then I’m old too. With all the talk anymore I guess I would check at least one cyl for clearance to make sure. I’m almost sure there’ll be more answers,.

 

[Ironbuilt]

You didn't state what the original CR was. If it was 10 to 1 and you are now increasing to 10.4 to 1, you are okay. If it was 11.5 to 1 and you are increasing to 11.75 to 1, you will have problems with pump gas.

 

[biomedtechguy]

Motor has stock type KB flat tops with valve reliefs.No cutting ever on block or heads.

You didn't state what the original CR was.

I'm thinking it should be the same as stock? 71 was 9.25 IIRC

 

[texas69bee]

I have those same pistons on a 383 and they sat down in the hole about .025. I ended up getting the blocked decked so the piston was at .004, used a .039 head gasket with the heads cc’d to 75 and still does fine on pump gas. If compression is what your worried about, your fine.

 

[66Satellite47]

No problem. The KB flat tops may sit a little higher in the hole compared to stock '71 pistons, but not by much. The KB valve reliefs will make sure the cam is not a problem. Go ahead do the steel shim gaskets, use KW Kopper Kote on them for a pretty good seal.

 

[Lefty71]

Motor has stock type KB flat tops with valve reliefs.No cutting ever on block or heads.The stock OEM cam had .450-.458 lift and came with steel gaskets.

The only super safe answer is "measure it", but with the info you provided, it doesn't sound like you have much to worry about. If you knew the compression distance on the KB pistons, then you already have everything in hand to mathematically eliminate (or confirm) a potential problem, but Im with the other answers..... id say go for it.

 

[popslavaz]

.

 

[popslavaz]

OK Got off the lazy train today,got out the tools and CC my parts. My stock (as far as I knew) 906 heads came in at 86 CC.And the piston at TDC +.040 down .011 in was 20CC.Checked all twice same both rounds. Sooo according to the compression chart on the web if I put everything in correctly,with the .020gasket I should be at a WHOPPING STOMPING 8.06-1. 390.85 CI POPS

 

[biomedtechguy]

WHOPPING STOMPING 8.06-1

I think you'll be OK...
Looking back on the early 60s Hot Rod articles at the New Orleans public library, in the mid to late 70s when I was in high school, I recall a 2 page "centerfold" of all of the motor offerings in their various levels of factory build.
I was already a Mopar fan (since 69)
The point is compression ratios were a BIG FACTOR in how they made horsepower then, so whatever fuel you have available, I say GET SUM CR!

 

[66Satellite47]

I'm thinking it should be the same as stock? 71 was 9.25 IIRC

Advertised CR are not accurate. That's why we really need to MEASURE stuff. IIRC Mopar was still using the .020 steel shim head gasket in '71. My figures for Pops combo is 8.8 CR w/.020 gasket. Seems reasonable. The -.040 deck seems a little deep, seems like most were about .030. Later versions of BB were more like the .040.

 

[66Satellite47]

OK Got off the lazy train today,got out the tools and CC my parts. My stock (as far as I knew) 906 heads came in at 86 CC.And the piston at TDC +.040 down .011 in was 20CC.Checked all twice same both rounds. Sooo according to the compression chart on the web if I put everything in correctly,with the .020gasket I should be at a WHOPPING STOMPING 8.06-1. 390.85 CI POPS

85/86 cc was the norm for all my un-cut 906's. Took a good cut to get them to the NHRA 80cc especially after I unshrouded the valves in the chamber. Compression makes HP & torque. I calculate 8.84 CR with a stock bore/stroke 383 .040 deck, 86 cc, flat top piston, .020 gasket. Seems like a 9.25 to 9.5 CR motor would run well on 92 pump premium.

 

[popslavaz]

The +.040 is my oversize bore, the down .011 is piston below deck POPS

 

[66Satellite47]

With a +.040 bore, stock stroke, -.011 deck, 86cc heads &.020 gasket you should have 9.5 CR

 

[wyrmrider]

you got lucky on the pistons
any idea what they are?
most aftermarket "builder" pistons are "destroked" for even further down the hole
KB's (not Silv-O-lite or Sealed Power) are full height (or better)

 

[texas69bee]

With a +.040 bore, stock stroke, -.011 deck, 86cc heads &.020 gasket you should have 9.5 CR

The piston has valve reliefs -8cc.
Doesn’t that take him to 9 CR?

 

[66Satellite47]

My mistake, with valve reliefs 8.9 -9.0. An 8 CC relief seems pretty big for a stock type piston.

 

[texas69bee]

My mistake, with valve reliefs 8.9 -9.0. An 8 CC relief seems pretty big for a stock type piston.

I’m pretty sure this is what he has. Some web sites advertise 5cc others advertise 8cc

 

[66Satellite47]

That pic shows a very large valve relief. It should handle a big cam. Looks like it could be 8 CC.

 

[moper]

You're fine. Use any gasket you want to. There's plenty of room.